Common-Input Ownership

Common-input ownership is the clustering heuristic that treats every input address in a single transaction as belonging to the same controlling entity.

On UTXO chains like Bitcoin, spending several inputs in one transaction requires signing with every corresponding key, so the entity building the transaction must control all of them. Common-input ownership formalizes this into the strongest clustering heuristic available: co-spent inputs are grouped into one entity. Applied transitively across history, it merges addresses into large, high-confidence clusters that anchor entity attribution. It is not infallible — CoinJoin and other collaborative transactions deliberately break the assumption — which is why intelligence systems flag such transactions and downgrade attribution confidence rather than blindly merging their inputs.